Growth Signals: Auxins and Tropisms Quiz

Auxin, most commonly in the form of indole-3-acetic acid (IAA), is a plant hormone produced mainly at the shoot tip or apical meristem. Its primary role is to promote cell elongation by stimulating the loosening of cell walls. Auxin controls the direction of plant growth in response to environmental stimuli such as light and gravity.

Phototropism occurs when light shining from one side causes auxin to migrate to the shaded side of the shoot. The higher auxin concentration on the shaded side promotes greater cell elongation there, causing the shoot to bend toward the light source. This response helps plants maximize their exposure to sunlight for photosynthesis.

When light strikes a shoot from one direction, auxin moves laterally away from the light source and accumulates on the shaded side. Higher auxin concentration stimulates more rapid cell elongation on that side, while the illuminated side elongates less. This unequal growth causes the shoot to curve toward the light in a response called positive phototropism.

Gravitropism, also called geotropism, is the directional growth response of plant organs to gravity. Roots exhibit positive gravitropism by growing downward in the direction of gravitational pull, while shoots exhibit negative gravitropism by growing upward against gravity. Auxin redistribution in response to gravitational signals plays a central role in mediating these responses in both shoots and roots.

Roots and shoots respond to gravity in opposite directions. Roots display positive gravitropism, meaning they grow downward in the direction of gravitational pull. Shoots display negative gravitropism, growing upward against gravity. This opposite response ensures that roots penetrate the soil to absorb water and nutrients while shoots grow toward sunlight for photosynthesis.

In phototropism, auxin migrates laterally from the lit side to the shaded side of the shoot. The shaded side has a higher auxin concentration, which stimulates greater cell elongation. As a result, the shaded side grows faster and longer than the lit side, causing the shoot to curve toward the light source. Auxin distribution is not equal during this response.

Auxin is primarily synthesized in the shoot apical meristem and in young, actively growing leaves at the tip of the plant. From there, it is transported downward through the plant in a process called polar auxin transport. This directional movement of auxin from the tip downward is what allows it to coordinate growth responses throughout the entire shoot.

Roots are more sensitive to auxin than shoots. When a root is placed horizontally, auxin accumulates on the lower side due to gravity. While this higher concentration promotes elongation in shoot cells, the same concentration inhibits elongation in root cells. As a result, the lower side of the root grows less, and the root curves downward, demonstrating positive gravitropism.

Statoliths are dense starch-filled organelles found in specialized cells called statocytes, located in the root cap. When a plant is reoriented, statoliths settle in the direction of gravity due to their density. This sedimentation is believed to trigger signals that redistribute auxin asymmetrically, initiating the gravitropic bending response that guides root growth downward.

Apical dominance is the process by which auxin produced at the shoot tip suppresses the growth of lateral buds lower on the stem. High auxin concentrations moving downward from the apex inhibit the development of side branches, ensuring the plant prioritizes upward growth. When the shoot tip is removed, auxin levels drop and lateral buds are released to grow, producing a bushier plant.

In shoots placed horizontally, auxin accumulates on the lower side and promotes greater elongation there, causing the shoot to bend upward, which is negative gravitropism. In roots, this same auxin accumulation inhibits cell elongation on the lower side, bending the root downward into the soil. Statoliths in root cap cells detect gravity and initiate the signal for auxin redistribution.

When the seedling is exposed to unidirectional light, the photoreceptor protein phototropin detects the light and triggers lateral movement of auxin away from the illuminated side. The shaded side accumulates more auxin and its cells elongate faster. This differential growth rate between the two sides of the shoot causes it to curve toward the light source.

Auxin does not uniformly promote cell elongation in all plant organs or at all concentrations. While moderate auxin concentrations promote elongation in shoot cells, the same concentrations can inhibit elongation in root cells due to the higher sensitivity of root tissue to auxin. This differential sensitivity is precisely what allows auxin to mediate opposite gravitropic responses in shoots and roots simultaneously.

Phototropin is the plant photoreceptor responsible for detecting blue light and triggering the phototropic response. When activated by blue light on the illuminated side of the shoot, phototropin initiates signaling pathways that cause lateral redistribution of auxin toward the shaded side. This directional auxin movement then drives the differential cell elongation that results in the shoot bending toward the light.

Phototropism (bending toward light), gravitropism (roots growing downward), and thigmotropism (growth in response to touch, such as vines coiling around supports) are all recognized plant tropisms involving directional growth responses to external stimuli. Flowers opening in response to temperature changes is a nastic movement, not a tropism, because it is non-directional and does not depend on the direction of the stimulus.